Metal oxides, in particular mixed metal oxides have a broad range of applications such as e.g. ceramics, polymer additives, fillers, pigments, reactive surfaces, catalysts, storage materials, polishing additives, membranes, fuel cells etc. Among the most important metal oxides are cerium oxide, cerium-zirconium and other stabilized zirconia mixed oxides, titanates and other mixed oxides below referred to as ceramic oxides. If these materials are used as nanoparticles (particle diameter below 200 nm), they exhibit advantageous properties such as high catalytic activity, improved processing capability, low sintering temperature, good dispersion capability, just to name a few. Titanates are used as dielectics for capacitors. Nanoparticles are of high interest as reduced feature size requires thinner sheets of dialectics and since such thinner sheets are preferable made from very small particles, for example, nanoparticles.
Current methods for the production of metal oxides are mechanical and mechanical/thermal processes, wet-phase chemistry based methods, and high temperature methods such as flame spray pyrolysis (FSP). For the latter, energy to drive the conversion to nanosized oxides can be radiofrequency (plasma), microwaves, laser or shock waves. Most convenient, however, is the use of thermal energy since in most cases this is the least expensive source of energy.
Mechanical and mechanical/thermal methods are energy intensive (milling!) and generally suffer from insufficient mixing at the atomic level leading to low phase stability and/or low specific surface area. Impurity from the milling (abrasion) reduces product purity and performance.
Wet-phase based methods entail huge solvent costs, produce large amounts of waste water and need calcination steps after the synthesis, making them cost intensive. Furthermore, although e.g co-precipitation of ceria/zirconia can lead to mixed oxide powders with extremely high specific surface areas, unfortunately, the temperature stability of as-prepared oxides is characterized by a big loss of specific surface area at elevated temperature. The same observation applies for most wet-phase made ceramics. Preparation at high temperature may produce an oxide with increased stability. This has prompted several people to attempt to prepare oxides by flame spray based methods. Flame spray pyrolysis (FSP) is a known process and has been used for preparation of many oxides. It uses thermal energy and has the inherent advantage of supplying low cost energy to drive nanoparticle formation. However, in the case of many oxides, the research for suitable precursors entails huge problems associated with the chemical properties of these compounds. For example Yoshioka et al. (1992) used FSP for the production of ceria oxides, but they received a powder of low specific surface area. WO 01/36332 discloses a FSP method leading to an inhomogeneous product comprising ceria particles of broadly varying sizes. Aruna et al. (1998) investigated the ceria/zirconia synthesis by combusting mixtures of redox compounds and oxidizing metal precursors. This high temperature preparation yielded a high surface area product with apparently good phase mixing in as-prepared powders. However, the preparation of ceramics by solid combustion is difficult to realize at high production rates, since the process may quickly run out of control. Furthermore it is basically a batch process and the reproducibility is a general problem. Laine et al. (1999) and Laine et al. (2000) used a spray pyrolysis unit to prepare ceramic oxides but the specific surface area of the product powder stayed low, at 10 to 16 m2/g. EP 1 142 830 also discloses a FSP method for the preparation of ceria/zirconia starting from organometallic compounds in organic solvents and/or water. The procedure disclosed in EP 1 142 830 focuses on chlorine free powders produced by flame spray pyrolysis and uses precursor solutions of type MeR where R is an organic rest such as methyl, ethyl, or a corresponding alkoxy-rest or a nitrate anion. As solvents, water or alcohols are used. U.S. Pat. No. 5,997,956 discloses a procedure where a liquid or liquid like fluid near its supercritical temperature is injected in a flame or plasma torch and thereby converted to nanoparticles.
WO 02/061163 A2 discloses an apparatus for the production of powders or film coatings. Thereby, the metal containing liquid is atomized without the use of a dispersion gas. Oljaca et al. (2002) describe a process using similar nozzles for the manufacture of nanoparticles. They only describe very low production rates with solutions being less than 0.05 M in metal. Droplet size distribution is stated as a major parameter for the successful nanoparticle synthesis. They report on the synthesis of yttria stabilized zirconia amongst others.
Recently Mädler et al. (2002B) disclosed an FSP method for the production of pure ceria with high surface and homogeneous particle sizes using a two phase nozzle to disperse the metal containing liquid by a dispersion gas (oxygen or air) and igniting the resulting spray by a premixed falame surrounding said nozzle. Such burner is furtheron in this document termed a spray burner. The solvent system used by Mädler et al., however, has now been found to be unsuitable for the production of e.g. ceria/zirconia. Stark et al. (2003) disclose the use of acetic acid and lauric acid for the preparation of ceria, zirconia and ceria/zirconia. Maric et al. (2003) use a not further specified CxH2zCeO6 precursor for the preparation of ceria, gadolinia and samaria doped ceria for fuel cell membranes. They applied a dispersion gas free atomization device working at low production rate and using a Nanomiser device (WO 02/061163 A2, see above) that makes very small droplets (below 10 micrometer). Overall production rate even using such a multiple nozzle setup is still below 1 kg/h.
In order to bring the nanoparticle manufacture from the pilot-scale production to an industrial scale synthesis (kg to ton quantities), some additional problems are to be faced. The most prominent is the choice of readily accessible metal precursors that allow sufficiently high production rates. The present invention links the manufacture of nanoparticles to existing metal containing products that were developed for different applications but not the manufacturing of nanoparticles. A second problem is production rate. Using multiple arrays as in WO 02/061163 A2 entails problems with maintenance, nozzle clogging, space, reproducibility and others. It would be much preferred to use few burners to make the same quantity of powder. It would further be of much use to apply a metal carrier liquid that can be sprayed on most conventional oil burners and does not require the sophisticated atomisation devices as e.g. in WO 02/061163. This further much helps scaling the production further up as oil burners with well above 100 kg oil/h are available. As it will become apparent within this invention, such a burner could achieve up to 20 kg ceramic particles per h (for 100 kg feed/hour).
For e.g. ceria, zirconia and ceria/zirconia all hitherto known methods use dilute metal solutions (usually <0.15 moles of metal/liter) resulting in low production rates. High metal concentrations are favorable as they directly increase the production rate of the process. Therefore, the metal concentration in the carrier liquid should be as high as possible. In the scope of the present invention, the flame spray process was found to limit the range of possible carrier liquid formulations by the viscosity as the liquid has to be dispersed during the process. While droplet size was found to be of minor importance, very viscous liquids could not be sprayed at all. It is therefore of high interest to find precursors for flame spray synthesis of oxide and metal nanoparticles that combine low viscosity and high metal concentration. Furthermore, such formulations should be readily produced and be stable upon storage. It is yet another objective of the present invention to show that common oil burners can be used for the synthesis of nanoparticles if the metal carrier liquid exhibits the above mentioned characteristics.